Environmental considerations preclude burning of fuels with high sulfur content, since the consequent production of sulfur dioxide presents a serious pollution problem. To overcome this, the sulfur in the fuel must be removed prior to, during or after combustion. For users of large quantities of fuel, such as utilities, removal of sulfur after combustion generally has been recognized as most feasible. In this regard, considerable research has been done and a number of processes for removal of sulfur dioxide from the combustion gases have been developed. Among these are lime and limestone scrubbing, magnesium oxide scrubbing, sodium scrubbing with thermal regeneration, e.g. see U.S. Pats. Nos. 3,477,815 and 3,485,581, sodium scrubbing with electrolytic regeneration, e.g. see U.S. Pat. No. 3,475,122, citrate process (U.S. Bureau of Mines (Report of Investigations 7774, (1973)), phosphate process (Stauffer Chemical (Chem. Eng. 81, July 8, pp. 46-47 (1974)), the double alkali process, the catalytic oxidation process to make sulfuric acid. These and other processes have been reviewed by A.V. Slack of Noyes Data Corp. ("Sulfur Dioxide Removal from Waste Gases" -- Pollution Control Review No. 4 (1971) by A.V. Slack; Noyes Data Corp.; Park Ridge, N.J. 07656).
Among the many considerations that enter into the decision of which type of process might be best are capital and operating costs, reliability, flexibility in operation, production of useful sulfur products, disposal of byproducts, etc.